Support and dispersion effects on the CO-H 2 reaction-on silica and alumina supported pt catalysts

Abstract

The CO-H 2 reaction was used as a catalytic probe to study the effect of pretreatment on support-metal interactions and Pt dispersion for a series of silica- and alumina-supported Pt catalysts. In general, it was found that pretreatment in an inert gas atmosphere as the temperature was linearly increased resulted in catalysts having higher Pt dispersions than catalysts pretreated in a H 2 atmosphere. For alumina-supported Pt catalysts, the effect of pretreatment had a marked effect on the adsorption of CO. The infrared spectrum of CO adsorbed on alumina-supported Pt treatment in He, suggested a very heterogeneous Pt crystal structure, as evidenced by three infrared CO stretching bands centered at 2062, 2050, and 2025 cm −1. In contrast, only one infrared CO stretching band centered at 2057 cm −1 was observed for alumina-supported Pt catalysts pretreated in H 2. CO adsorption on silica-supported Pt catalysts was insensitive to pretreatment conditions. This is explained in terms of a possible orienting influence attributed to the alumina support. The influence of dispersion and pretreatment schedule on the CO + H 2 reaction was small. However, slightly larger variations in rate with changes in dispersion were observed on the Pt-Al 2O 3 catalysts. Higher turnover numbers for methane formation were observed on the larger Pt crystallites. Support effects on the CO + H 2 reaction were considerably larger than dispersion effects. Turnover numbers for methane formation on the alumina-supported Pt catalysts were half an order of magnitude greater than those observed on the silica-supported Pt catalysts. This is explained in terms of decreased C-O bond strength. Increases in surface heterogeneity had little effect on the rate of the CO-H 2 reaction.